Severe acute myositis and myocarditis on initiation of 6-weekly pembrolizumab post-COVID-19 mRNA vaccination

We describe three cases of critical acute myositis with myocarditis occurring within 22 days of each other at a single institution, all within 1 month of receiving the initial cycle of the anti-PD-1 drug pembrolizumab. Analysis of T cell receptor repertoires from peripheral blood and tissues revealed a high degree of clonal expansion and public clones between cases, with several T cell clones expanded within the skeletal muscle putatively recognizing viral epitopes. All patients had recently received a COVID-19 mRNA booster vaccine prior to treatment and were positive for SARS-CoV2 Spike antibody. In conclusion, we report a series of unusually severe myositis and myocarditis following PD-1 blockade and the COVID-19 mRNA vaccination.

placed on this supplemental material which has been supplied by the author(s)  S2) and skeletal muscle biopsies as labelled (N.B. repertoires found in inflammatory myositis biopsies and control biopsies came from Montagne et al 18 ).(B) as for (A) but MH index for TCR repertoire overlap between PM cardiac muscle from patient 3 and muscle biopsies as labelled.(C) MH index showing TCR repertoire overlap between the PM cardiac muscle from patient 3, the PM tumour from patient 3 and resected melanomas from eight patients pre-2020 (table S2).
BMJ Publishing Group Limited (BMJ) disclaims all liability and responsibility arising from any reliance Supplemental material placed on this supplemental material which has been supplied by the author(s) J Immunother Cancer doi: 10.1136/jitc-2023-008151 :e008151.12 2024; J Immunother Cancer , et al.

Supplemental Data
Supplementary report 1full histopathology report of skeletal muscle biopsy from patient 1

MICROSCOPIC REPORT:
Frozen sections show skeletal muscle with a significant increase in the variability of muscle fibre diameters (approximate range: 11-96 µm).There are atrophic and some hypertrophic fibres.The main pathology is patchy and characterised by prominent multifocal clusters of pale necrotic and basophilic regenerating fibres associated with loose macrophage-rich inflammatory infiltrates.There is a focal increase in internal nuclei, which is likely secondary.There is a very focal increase in endomysial connective tissue associated with the areas of necrosis / regeneration.The blood vessels appear unremarkable.Paraffin-embedded material adds no further information.

Special Stains
Modified Gomori trichrome shows occasional vacuolated fibres, which appear to be necrotic.
There are no convincing rods or ragged red fibres.The overall lipid content of muscle fibres is within normal limits.Rare fibres show a significant increase (significance uncertain).The glycogen content of muscle fibres is not increased.Oxidative enzyme analysis shows a small number of cytox negative / SDH positive fibres (likely secondary).There are no cores or targets.

Immunohistochemistry
The fast (type 2) fibres comprise 60-70% of fibres.There is clustering but no significant grouping of fibre types.Atrophic fibres are of both types.Scattered fibres (mostly atrophic) are positive for fetal myosin, a proportion of which are also positive for embryonic myosin consistent with regeneration.A significant number of fibres are positive for NCAM.There is strong patchy sarcolemmal and sarcoplasmic upregulation of MHC-1.LCA and PGM-1 highlight the macrophage-rich inflammatory infiltrates associated with clusters of necrosis / regeneration.C5b-9 stains necrotic fibres diffusely, as expected.In addition, there is capillary deposition.p62 shows a spectrum of diffuse and granular staining in a small number of fibres (non-specific).Spectrin highlights the variability of fibre diameters.
COMMENT: The clinical diagnosis of pembrolizumab-associated myositis is noted.Pembrolizumab (monoclonal antibody, immune checkpoint (PD-1) inhibitor) was administered as part of systemic melanoma therapy.This muscle biopsy shows features which are entirely supportiveof immune checkpoint inhibitor-associated myositis (ICIAM), as described in the literature (e.g.Shelly et al Brain Communications 2020, doi:10.1093/braincomms/fcaa181). In particular, the multifocal clusters of necrotic fibres as opposed to single scattered necrotic / regenerating fibres typically observed with immune-mediated necrotising myopathies (IMNM) are a characteristic finding in ICIAM.Cytox negative SDH positive fibres have also been described as a secondary feature.Close correlation with the ongoing clinical and any other relevant findings is advised.

DIAGNOSIS: MUSCLE BIOPSY -FEATURES SUPPORTIVE OF IMMUNE CHECKPOINT IN-HIBITOR (PEMBROLIZUMAB) -ASSOCIATED MYOSITIS (ICIAM). PLEASE SEE COMMENT.
BMJ Publishing Group Limited (BMJ) disclaims all liability and responsibility arising from any reliance Supplemental material placed on this supplemental material which has been supplied by the author(s) Figure S5.(A) Morisita-Horn (MH) index for the TCR repertoire overlap between resected melanomas from eight patients from the original cohort (tableS2) and skeletal muscle biopsies as labelled (N.B. repertoires found in inflammatory myositis biopsies and control biopsies came from Montagne et al 18 ).(B) as for (A) but MH index for TCR repertoire overlap between PM cardiac muscle from patient 3 and muscle biopsies as labelled.(C) MH index showing TCR repertoire overlap between the PM cardiac muscle from patient 3, the PM tumour from patient 3 and resected melanomas from eight patients pre-2020 (tableS2).